isa: Clean up error handling around isa_bus_new()

We can have at most one ISA bus.  If you try to create another one,
isa_bus_new() complains to stderr and returns null.

isa_bus_new() is called in two contexts, machine's init() and device's
realize() methods.  Since complaining to stderr is not proper in the
latter context, convert isa_bus_new() to Error.

Machine's init():

* mips_jazz_init(), called from the init() methods of machines
  "magnum" and "pica"

* mips_r4k_init(), the init() method of machine "mips"

* pc_init1() called from the init() methods of non-q35 PC machines

* typhoon_init(), called from clipper_init(), the init() method of
  machine "clipper"

These callers always create the first ISA bus, hence isa_bus_new()
can't fail.  Simply pass &error_abort.

Device's realize():

* i82378_realize(), of PCI device "i82378"

* ich9_lpc_realize(), of PCI device "ICH9-LPC"

* pci_ebus_realize(), of PCI device "ebus"

* piix3_realize(), of PCI device "pci-piix3", abstract parent of
  "PIIX3" and "PIIX3-xen"

* piix4_realize(), of PCI device "PIIX4"

* vt82c686b_realize(), of PCI device "VT82C686B"

Propagate the error.  Note that these devices are typically created
only by machine init() methods with qdev_init_nofail() or similar.  If
we screwed up and created an ISA bus before that call, we now give up
right away.  Before, we'd hobble on, and typically die in
isa_bus_irqs().  Similar if someone finds a way to hot-plug one of
these critters.

Cc: Richard Henderson <rth@twiddle.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Hervé Poussineau" <hpoussin@reactos.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1450370121-5768-11-git-send-email-armbru@redhat.com>